Three stage drawing process for stereospecific polypropylene to give high tenacity filaments



1954 H. D. NOETHER ETAL 3, 6 ,709

THREE STAGE DRAWING PROCESS FOR STEREOSPECIFIC POLYPROPYLENE TO GIVE HIGH TENACITY FILAMENTS Filed Nov. 21. 1960 TENACITI, cams/11mm EIDNGATION,

United States Patent The present invention relates to the preparation of stereospecific polypropylene filamentary material.

Polyolefin filamentary materials have found limited application because of their pronounced tendency to extend excessively or creep upon prolonged application of loads even far below their breaking values.

It is an object of the present invention to produce polypropylene filamentary materials exhibiting high tenacity, substantially no post yield ductility and substantially unchanged length even upon prolonged loading.

Other objects and advantages of the invention will become apparent from the following detailed description of the invention wherein all parts are by weight unless otherwise expressed.

In accordance with one aspect of the invention stereospecific polypropylene is dissolved in a volatile solvent and the solution is extruded through a plurality of orifices into an evaporative atmosphere to produce a plurality of filaments. The filaments are collected and the resulting bundle, e.g., a yarn, is taken up at a linear speed faster than that at which the solution left the orifices, i.e., the filaments are drawn down or stretched. This yarn is then subjected to second and third drawings. The second drawing is carried out at a fiber temperature of about room temperature to 110 C. at a draw ratio of about 2:1 to 10:1 and preferably at a fiber temperature of about 60 to 100 C. at a draw ratio of about 4:1 to 8:1, the tenacity being raised thereby to in excess of about grams per denier. The third drawing is carried out at a fiber temperature of about 140 to 150 C. at a draw ratio of about 1.1:1 to 4:1 and preferably at a fiber temperature of about 147 C. at a draw ratio of about 1.8:1 to 3.5 :1. Advantageously the total draw ratio in the second and third drawings is at least about 9:1 and preferably at least about 12:1, the tenacity being raised thereby to in excess of about 8 grams per denier while the elongation exceeds and often 15%. The stress strain behavior of the yarn shows substantially no post yield ductility, i.e., the stress-strain curve is a relatively straight line.

The drawing temperatures recited hereinabove have reference to the temperature of the filaments which will usually be lower than the temperature of the drawing apparatus. For example, if a filament is to be drawn at 145 C. and the medium is hot air, it may be necessary to heat the air to 150 to 155 C. for the filament to reach the desired temperature in the short time it traverses the hot air Zone; conversely, if the air temperature is 145 C. the filament temperature may be lower.

The starting material in accordance with the present invention comprises stereospecific polypropylene such as is produced by polymerization in the presence of the usual type of heterogeneous catalyst such as titanium trior tetra-chloride plus an aluminum alkyl or in the presence of chromium oxide-alumina-silica catalyst. Minor amounts of copolymerizable monomers may also be present. The product, usually called isotactic polypropylene, has a density ranging from about 0.88 to 0.92 gram per cubic centimeter and a melting point exceeding 165 C. Its inherent viscosity usually exceeds about 1.0 and gen- 3,161,709 Patented Dec. 15, 1964 ICC erally ranges from about 1.5 to 4.0 as determined in decalin at C.

The polymer can be converted into filamentary material by dry spinning as described above or by wet or melt spinning, as well as by various combinations thereof. While the filamentary material is usually attenuated during production it may be pulled along at a speed equal to or only a fraction of the extrusion speed. It should, however, be prevented from uncontrolled contraction and shrinkage and such positive control constitutes the first drawing step. The filamentary material may constitute a monofilament or a bundle of filaments, e.g., a bundle of relatively few filaments such as a yarn or of very many filaments such as a tow which is ultimately intended for conversion into staple fiber, or the like.

The second and third drawings may be carried out sequentially with the production of the starting yarn and/ or with one another. While hot fluids such as water or air may be employed to bring the yarn up to drawing temperatures, hot rolls, pins, shoes or plates may also be employed either by themselves or in conjunction with one another.

The invention will be further described with reference to the accompanying drawing wherein:

Curve A is the stress-strain curve for an isot'actic polypropylene which has been spun into yarn and then drawn;

Curve B is the stress-strain curve resulting from further drawing of the yarn of curve A; and

Curve C is the stress-strain curve of a commercial polypropylene yarn.

From a comparison of curves A and B it can be seen that the product of curve B is far stronger than that of curve A. Curve B is far closer to a straight line than curve A and it is free of a sharp knee, which is a graphic representation of the low post yield ductility of the novel product, i.e., it does not commence elongating rapidly once the load exceeds some special value. The commercial material represented by curve C also has a low post yield ductility but its tenacity at break is less than half that of the novel product. From the similarity between curve C and the initial portion of curve A it appears that the product of curve C is the same as that of curve A except for some treatment which reduces its elongation without improving its tenacity.

The post yield ductility can be evaluated by comparing the actual elongation at break tenacity with the elongation in excess of that obtained by extending the straight line portion of the stress-strain curve (curve B) to the break tenacity (curve B), i.e., the ratio of YX to ZX. With the novel products of this invention the ratio is less than about 0.7 and is often less than 0.6.

The novel product resulting from the three stage drawing exhibits an X-ray diagram the sharpness of which is equal to or greater than that of a product only twice drawn. However, its overall birefringence as measured by the Becke line method is about 0.035 or higher as contrasted with about 0.032 for the same material drawn but twice.

The thrice-drawn products are also characterized by marked stability or resistance to creep under load, e.g., under a load of 2 grams per denier they are extended usually less than about 6% even after 1200 hours. For

the same length of time 3 grams per denier extend the Example (a) Commercial isotactic polypropylene having an inaerent viscosity of 2.5, as measured in decalin at 135 3., and a melting point of 168 C. is dissolved to a con- :entration of 24% in xylene. The solution is extruded hrough 5 circular orifices each 0.076 mm. in diameter nto hot air at 100 C. The resulting filaments are )ulled by a first pair of draw rolls driven at a peripheral speed of 10 meters per minute and wound onto a bob- )in with 0.3 turn per inch. The 98 denier product has l tenacity of about 1.5 grams per denier and an elongaion of about 500%; the large variability between suc- :essive samples makes a precise determinaton impossiale.

(b) The bobbin is removed, the yarn is advanced over t pin immersed in water at 93 C. onto a draw roll op- :rating at a peripheral speed such that the yarn is drawn 6.6 times its length and the yarn is collected on bobins. It has a tenacity of 6.03 grams per denier and an longation of 37.7%, its stress-strain curve being shown [1 the drawing as curve A. Under a constant load of 2 rams per denier in 1,000 hours the yarn is extended 5% and in 1500 hours it breaks at an extension of 5%. Under a constant load of 4 grams per denier the am breaks in 2 minutes.

(c)-The yarn drawn as in (b) is then redrawn over hot shoe 30 centimeters long and maintained at a temerature of 146-148 C. as measured by a thermocouple laced under the shoe surface. The yarn is fed to the me at 1.4 meters per minute and is drawn away from 1e shoe at 3.4 meters per minutes, i.e., a draw ratio of .421. The yarn has a denier of 5.9, a tenacity of 10.4 rams per denier, an elongation of 13.7% and a post ield ductility of about 0.56. Its stress-strain diagram shown in the drawing as curve B. Under a constant )ad of 2 grams per denier in 1200 hours the yarn is ex- :nded 4%. Under a constant load of 4 grams per deier the yarn does not break even after 1,200 hours at 'hich time it has extended 8%. Under a constant load f 7 grams per denier the yarn does not break even in 00 hours, at which time it has extended 14%.

It is to be understood that the foregoing detailed dezription is given merely by way of illustration and that [any variations may be made therein without departing 'om the spirit of our invention.

Having described our invention what we desire to seire by Letters Patent is:

1-. The process which comprises extruding stereospelfic polypropylene in liquid state through an orifice to reduce a filament, drawing said filament continuously om said orifice, drawing said filament a second time at temperature of about room temperature to 110 C. 1d a draw ratio ranging from about 2:1 to 10:1, and 'awing said filament a third time at a temperature of mm 140 to 150 C. and a draw ratio ranging from about 1:1 to 4: 1.

2. The process set forth in claim 1, wherein said polyfopylene is extruded in the form of a melt.

3. The process set forth in claim 1, wherein said polyopylene is extruded in the form of a solution.

4. The process set forth in claim 1, wherein the comned draw ratio of said second and third drawing steps at least about 12:1.

5. The process set forth in claim 1, wherein the first rd second drawing steps are carried out to impart to e product of the second drawing a tenacity of at least out 8 grams per denier.

6. The process which comprises extruding stereospecific polypropylene in liquid state through an orifice to produce a filament, drawing said filament continuously from said orifice, drawing said filament a second time at a temperature of about 60 to C. and a draw ratio ranging from about 4:1 to 8: 1, and drawing said filament a third time at a temperature of about 140 to 150 C. and a draw ratio ranging from about 1.8:1 to 3.5 :1, the first and second drawing steps being carried out to impart t0 the product of the second drawing a tenacity of at least about 5 grams per denier, and the combined draw ratio of said second and third drawing steps being at least about 12:1.

7. The process which comprises drawing a fully-formed filament of stereospecific polypropylene at a temperature of about room temperature to C. and a draw ratio ranging from about 2:1 to 10:1, and further drawing said filament at a temperature of about to C. and a draw ratio ranging from about 1.1:1 to 4:1.

8. The process set forth in claim 7, wherein the combined draw ratio of said drawing steps is at least about 12:1.

9. The process set forth in claim 7, wherein the polypropylene subjected to said further drawing step has a tenacity of at least about 5 grams per denier.

10. The process which comprises drawing a fullyformed filament of stereospecific polypropylene at a temperature of about room temperature to 110 C. and a draw ratio ranging from about 4:1 to 8:1, and further drawing said filament at a temperature of about 140 to 150 C. and a draw ratio ranging from about 1.811 to 3.5 :1, the polypropylene subjected to said further drawing step having a tenacity of at least about 5 grams per denier and the combined draw ratio of the second and third drawing steps being at least about 12:1.

11. The process which comprises drawing at a temperature of about 140 to 150 C. and at a draw ratio of at least about 11:1 a filament of stereospecific polypropylene having a tenacity of at least about 5 grams per denier.

12. The process which comprises drawing at a temperature of about 147 C. and at a draw ratio of about 1.811 to 3.5:1 a filament of stereospecific polypropylene having a tenacity of at least about 5 grams per denier.

References Cited in the file of this patent UNITED STATES PATENTS 2,325,060 Ingersoll July 27, 1943 2,715,763 Marley Aug. 23, 1955 2,807,863 Schenker Oct. 1, 1957 2,816,349 Pamm et al Dec. 17, 1957 2,846,752 Lessig Aug. 12, 1958 2,988,783 Miller et al. June 20, 1961 3,007,765 .Ruyter Nov. 7, 1951 3,059,991 Munt Oct. 23, 1962 FOREIGN PATENTS 570,390 Belgium Aug. 30, 1958 597,887 Canada May 10, 1960 1,024,201 Germany Feb. 13, 1958 1,040,179 Germany Oct. 2, 1958 1,184,613 France Feb. 9, 1959 OTHER REFERENCES Golding: Polymers and Resins, pp. 653-655; published in 1959 by D. Van NQstrand. i 

1. THE PROCESS WHICH COMPRISES EXTRUDING STEREOSPECIFIC POLYPROPYLENE IN LIQUID STATE THROUGH AN ORIFICE TO PRODUCE A FILAMENT, DRAWING SAID FILAMENT CONTINUOUSLY FROM SAID ORIFICE, DRAWING SAID FILAMENT A SECOND TIME AT A TEMPERATURE OF ABOUT ROOM TEMPERATURE TO 110*C. AND A DRAW RATIO RANGING FROM ABOUT 2:1 TO 10:1, AND DRAWING SAID FILAMENT A THIRD TIME AT A TEMPERATURE OF ABOUT 140 TO 150*C. AND A DRAW RATIO RANGING FROM ABOUT 1.1:1 TO 4:1. 